Radio frequency (RF) transmitters are used to transmit RF signals over the air (or some other transmission medium, such as coaxial cable or other waveguide) to an RF receiver. To compensate for attenuation of the RF signals as they propagate to the receiver, the RF signals are amplified by a radio frequency power amplifier (RFPA), prior to being transmitted.
Various emerging and future military and commercial applications require or will require RFPAs capable of producing very high RF output powers, for example, tens to hundreds of watts, at microwave frequencies. Over the years, a substantial amount of research has been dedicated to identifying semiconducting materials that can be used to build RFPAs that satisfy this dual requirement of high-power and high-frequency. One of the most promising semiconducting materials that has been identified is gallium nitride (GaN). GaN is a group III/V semiconductor having a very wide bandgap (˜3.4 eV @ 300K) and a very high breakdown field (300 V/μm @ 300K). These two attributes are highly desirable since they afford the ability to manufacture GaN-based transistors with high breakdown voltages—a necessary requirement for realizing high RF output powers at good efficiency. GaN also has a high thermal conductivity (˜2.3 W/cm·K @ 300K), which further facilitates high power operation.
In order for semiconductor-based RFPAs to be capable of operating at microwave frequencies, the semiconducting material should also have a high carrier mobility. GaN in its bulk form has a moderate carrier mobility similar to that observed in silicon. However, the electron mobility can be substantially increased when GaN is used in a high electron-mobility transistor (HEMT) device topology. FIG. 1 is a simplified cross-sectional drawing of a typical GaN-HEMT 100, highlighting the GaN-HEMT's salient material layers and physical characteristics. The GaN-HEMT 100 includes an AlGaN/GaN heterostructure 102 formed on an electrically-insulating or semi-electrically-insulating substrate 104 (e.g., silicon-carbide (SiC), sapphire (Al2O3) or silicon (Si)). The different bandgaps of AlGaN and GaN result in formation of a quantum well in the lower-bandgap GaN 106 material, near the AlGaN/GaN interface. When the AlGaN/GaN heterostructure 102 is formed, charge carriers (i.e., electrons) from the wider bandgap AlGaN layer 108 diffuse into the quantum well in the lower bandgap GaN layer 106, thereby forming a highly-concentrated two-dimensional electron gas (2 DEG) 110. Confining the electrons to the 2 DEG 110 has the effect of substantially increasing the GaN electron mobility compared to what is observed in bulk GaN, making the GaN-based HEMT 100 suitable for high-frequency operation.
The high-power, high-frequency capability of the GaN-based HEMT has made it a desirable candidate for building RFPAs that are capable of operating at high frequencies and high RF output powers. In recent years, RFPAs utilizing GaN-based HEMTs have been successfully manufactured, validating this capability. However, methods and apparatus for efficiently driving GaN-HEMT-based RFPAs in wideband, high power applications are lacking and greatly needed.